1. Field of the Invention
The present invention relates to a joining element, made of thin sheet metal, for the concealed joining of a wooden component such as a beam or a post to a second component made of wood or another material. In particular the present invention is for a concealed beam attachment or a post foot, and has a planar attachment web provided with through holes which is to be inserted into a slot, generally on the end surface of the wooden component to be attached. The present invention is fastened or held with fasteners such as dowels or the like which, in the assembled state, extend through the through holes. At least two fastening flanges are provided on the present invention, each provided with through holes, and are generally bent 90 degrees from the plane of the attachment web and configured integrally with the attachment web. Each of the webs are laterally spaced away from the rear edge, which forms the fold or bending line between the attachment web and the fastening flanges and lies in a first main axis of the joining element of the attachment web, such that a second main axis of the joining element, lying at right angles to the first main axis and in the plane of the fastening flanges, extends substantially centered between the outer edges of the outer fastening flanges which extend perpendicular to the fold or bending line.
2. Discussion of the Prior Art
In wooden construction, a concealed joint is understood to be a joint in which the joining element, also referred to as a "fitting," is at least substantially surrounded by the wooden components. In a completely concealed joint, in which even the receiving slot is not visible externally, the slot is configured not as a continuous slot but as a "blind slot," in order to weaken the respective wooden component as little as possible and to prevent the slot from being visible after assembly. Available for this purpose are "keyway cutters," although their smallest slot width is 7 mm. Continuous slots are recessed automatically, generally with a circular saw, into the end surface of a beam. The minimum thickness of circular saw blades of a size suitable for common beam dimensions is also similar to the aforesaid cutting width.
Since the thickness of the attachment web of a joining element must, according to applicable specifications, be no more than 1 mm less than the slot width in order to prevent the attachment web from "wobbling" in the slot, the sheet thickness of such joining elements has previously been generally at least 6 mm. Entirely disregarding the fact that at this thickness the joining elements are generally considerably overdimensioned in terms of strength and moreover require an unnecessary expenditure of material, it is known that thicker sheets are more difficult and complex to work. For reasons of cost effectiveness, therefore, it would be desirable to reduce significantly the material thickness of the joining elements under discussion here, and embody them, for example, with a sheet thickness of only up to 3 mm.
With this purpose, it has already been proposed in German Utility Model 90 01 067 to manufacture a joining element from a thin sheet approximately 3 mm thick, and to provide its attachment web with shaped-in knobs or beads whose height is substantially equal to the slot width of the wooden component being attached. Although the risk of wobbling in a slot at least twice as wide can be reduced with a fitting of this kind, nevertheless with such thin sheet metal the number of through holes, and the number of fasteners to be installed in them, must be correspondingly large, since otherwise the permissible hole face pressure in the through holes will be exceeded.
German Utility Model 92 05 490 has proposed a thin-sheet perforated fitting in which the through holes of the attachment web are crimped over at their edges, preferably in groups, to one or the other side of the attachment web, such that the sum of the sheet thickness and the height of the crimps is slightly less than the predefined slot width of the wooden component being attached. This configuration has proved very successful, since this feature on the one hand results in a considerable increase in the effective thickness of the thin attachment web, which counteracts or prevents wobbling, and on the other hand results in a considerable increase in the contact surface of the fastening means in the through holes, with a corresponding decrease in hole face pressure. However, it has been found that, particularly when high stresses are exerted on the perforated sheet-metal fitting known from German Utility Model 92 05 490, the fastening flange adjacent to the attachment web can shift inward in the region of the slot into the wood of the wooden component being attached. The reason for this is that the fastening flange laterally adjacent to the attachment web is too narrow, or is present only at one side of the attachment web. Although in this previously-known perforated sheet metal fitting the other fastening flange extends symmetrically on either side of the first main axis and thus on either side of the attachment web, in the known perforated sheet-metal fitting this fastening flange necessarily lies above the right-angle attachment web and cannot always prevent the aforesaid negative effects, at least under extreme stress.
German Patent No. 41 24 553 has disclosed a metal connector for concealed beam attachments which is constituted by two thin-walled sheet-metal parts which are configured with mirror symmetry with respect to the plane of symmetry of the attachment web, specifically, two shaped panels each folded or bent at right angles, such that one folded section forms a fastening flange, and the other section forms part of the attachment web. The latter section is in each case meander-shaped, such that contact sections alternate with substantially groove-shaped curved sections. The groove-shaped curved sections, together with the respective corresponding curved section of the other shaped element, form a channel whose thickness is slightly less than the slot width.
It is immediately evident that this known joining element is complex due to the fact that it consists of two parts, and its installation is thus also relatively difficult.
A feature common to all previously known joining elements is that they are intended and suitable only for beam attachments, so that ordinarily other elements or apparatus must be used to support posts. Reference in this regard is made, for example, to a post foot according to German Utility Model 93 12 522.